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ATLAS at LHC

ATLAS at LHC. Helio Takai takai@bnl.gov Brookhaven National Laboratory (for the ATLAS collaboration). Quarkmatter 2004 Oakland, January 11-17,2004.

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ATLAS at LHC

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  1. ATLAS at LHC Helio Takai takai@bnl.gov Brookhaven National Laboratory (for the ATLAS collaboration) Quarkmatter 2004 Oakland, January 11-17,2004

  2. S. Aronson, K. Assamagan, B. Cole, M. Dobbs, J. Dolejsi, H. Gordon, F. Gianotti, S. Kabana, S.Kelly, M. Levine, F. Marroquin, J. Nagle, P. Nevski, A.Olszewski, L. Rosselet, H. Takai, S. Tapprogge, A. Trzupek, M.A.B. Vale, S. White, R. Witt, B. Wosiek, K. Wozniak and … ATLAS Heavy Ion Working Group xFTE ~ 1/2000 = 5x10-4 It has been active for ~2yrs and ~1year of simulation studies. Prefer full simulations due to complexity. Submission of LoI to LHCC by end of February.. Constraint: No modifications to the detector, with the exception of forward instrumentation

  3. MOTIVATION RHIC experiments have shown a suppression of high pT hadrons - jet quenching Interest in learning more about QCD phase transition. ATLAS is a detector designed for high pT physics and in particular has excellent jet capabilities. Growing interest within ATLAS to study questions related to astroparticle physics.

  4. The Quenching Picture

  5. What changes from RHIC to LHC? Parton kinematics, gluon density, increase in hard scattering cross section.

  6. http://atlas.ch

  7. ATLAS in Numbers Designed to operate at full LHC luminosity of 1034 Coverage Calorimeter -4.9<h<4.9 Muon Spectrometer -2.7<h<2.7 Inner Tracker -2.5<h<2.5 Calorimeter Segmentation EM Liquid Argon Calorimeter is finely segmented Hadronic Tile Calorimeter is also segmented Muon Spectrometer Tracking volume behind calorimeter Inner Detector Composed of Pixel, SCT and TRT Multi-level trigger and DAQ

  8. SIMULATIONS Simulation studies are all full simulations using a full detector description in Geant-3. Event Generator - HIJING, no quenching, dN/dh~3500. Simulations are divided into two h regions: |h|<3.2, and 3.2<h<4.9. It takes about 6h per event and uses the same parameters as in pp with the exception of calorimeter where the energy threshold is raised to 1 MeV.

  9. one b=0 collision in ATLAS simulation

  10. EVENT CHARACTERIZATION Collision centrality, Impact parameter determination Multiplicity and dET/dh dN/dh

  11. IMPACT PARAMETER Use 3 detector systems to obtain impact parameter, Pixel Detector, EM calorimeter and Hadronic Calorimeter All three systems have similar performance with impact parameter resolution of ~1fm

  12. TRACKING Tracking in ATLAS is accomplished by using 2 of the 3 inner detector subsystems (11 measurements): Pixel with occupancy <2% SCT with occupancy <20% The TRT occupancy is too large (although it will be available for pA) Resolution Efficiency and fakes (pT>1GeV)

  13. JETS (Pb+Pb) HIJING (0<b<1 fm) Background: ~2 GeV per 0.1x0.1 tower in EM ~0.2 GeV per 0.1x0.1 tower in HAD Soft hadrons ~ completely stop in EM The largest background is in 1st layer 0.1x0.1 cell ~20 GeV in a cone Threshold for jet reconstruction ~30 GeV Compare to full luminosity pp ~15 GeV 0.1x0.1 tower Reconstruction: Sliding window algorithm with splitting/merging after background subtraction (average and local) Algorithm is not fully optimized. -3.2<h<3.2

  14. JET SIMULATION

  15. JET SIMULATION sh=0.045 sf=0.035 (In heavy ion environment, 70 GeV ) Simulation indicate good performance for jet energy resolution, but for jets of transverse energy above ~40 GeV. Jet pointing resolution is also preserved. Energy calibration - Developed for the ATLAS detector based on H-1 algorithm.

  16. JET RATES ATLAS accepted jets for central Pb-Pb Jet pT > 50 GeV 30 million ! Jet pT > 100 GeV 1.5 million Jet pT > 150 GeV 190,000 Jet pT > 200 GeV 44,000 Vitev - extrapolated to Pb-Pb Every accepted jet event is an accepted jet-jet event since ATLAS has nearly complete phase space coverage ! g-jet106 events/month with pT >50 GeV g and Z0 have no radiation ! g*-jet10,000 events/month with pT >50 GeV with g* m+m- Z0-jet 500 events/month with pT >40 GeVwith Z0m+m-

  17. HEAVY QUARKS Radiative quark energy loss is qualitatively different for heavy and light quarks. (WH events overlapped on top of HIJING) b-jet tagging efficiency by dislocated vertex seems to be possible in the heavy ion environment. Muon tagging should improve b-jet tagging Simulation

  18. THREE JET EVENTS? In 3jet events the “third” jet is a hard radiated gluon. Because the Quark-gluon plasma is a colored object, the gluon should couple much stronger to the media and therefore quenching should be larger. What should be observe? Perhaps an enhancement in the ratio of 2 to 3 jet events? obviously a LEP event!

  19. WHERE ARE WE HEADING WITH JETS ? Jet ET should be balanced - either in jet+jet, 3 jet, or g+jet, etc. - Conservation of Energy Fragmentation function, angular distribution should be sensitive to quenching - require definition of jet energy and axis based on energy flow Our goal is to reconstruct jets using calorimeter information and inner detector tracking to obtain information such as dN/dz, dN/dq, etc

  20. Jet ET Core Isolated neutral cluster within Jets - Low yield Jet Core ET - ET in a narrow cone around jet axis (DR=0.1).

  21. Jet Reconstruction (PRELIMINARY) Jet ET vs pT sum (of tracks) pT/ET, Pb+Pb and PYTHIA LOTS of work required!!!

  22. QUARKONIA SUPRESSION overlay upsilon decays on top of HIJING events. Single Upsilons HIJING background Half ’s from c, b decays, half from π, K decays for pT>3 GeV. Background rejection based on 2 cut, geometrical cut and pT cut. Dh, Df=difference between ID and µ-spectrometer tracks after back-extrapolation to the vertex for the best 2 association.

  23. UPSILON RECONSTRUCTION Stand alone muon spectrometer gives a mass resolution of ~460 MeV. Reconstruction uses track match to inner detectors. Barrel only (|h| <1) Acceptance 8.7% (cf full: 22.0%) +efficiency Resolution 126 MeV (152 MeV) S/B 2.0 (0.9) Purity 94-99% (91-95%) depends on the cuts

  24. UPSILON RECONSTRUCTION A compromise has to be found between acceptance and resolution to clearly separate upsilon states with maximum statistics (e.g. 10%)

  25. UPSILON RECONSTRUCTION A di-muon trigger using a m pT cut < 4 GeV is being investigated. A J/ study is also under way. smass=53 MeV =>easy separation of J/ and ’ Low mass =>decay ’s need an extra pT from the J/ or a Lorentz boost to get through the calorimeters. =>full pT analysis possible only forward and backward where the background is maximum.

  26. p-A physics Study of the modification of the gluon distribution in the nucleus at low xF. xg(x) enhanced by A1/3~6 in Pb compared to p kinematical access xF>10-5 Link between pp and AA physics Study of the jet fragmentation function modification pQCD in nuclear environment Occupancy in p+Pb is lower than in full luminosity pp. Full detector capabilities will be available. L~1030 translates to about 1MHz interaction rate (compare to 40 MHz in pp)

  27. Summary • ATLAS has an excellent calorimeter/muon-spectrometer coverage suitable for high-pT heavy-ions physics • jet physics (quenching) looks very promising • Upsilon is accessible • Pixel+SCT work in Pb-Pb collisions (tracking, particle multiplicities from hits) •  p-A, Ultra-Peripheral Collisions (easier than Pb-Pb collisions) will be studied

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